1. Field of the Invention
The present invention relates to an engine in which fuel is directly injected into a combustion chamber, that is, a "cylinder direct injection engine".
2. Description of the Related Art
A direct injection engine in which fuel is directly injected into the combustion chamber injects the fuel in a compression stroke under operating conditions of a low required load so as to enable stratified combustion forming an air/fuel mixture only at the periphery of a spark plug. In this case, a throttle valve is not throttled, therefore a pumping loss can be reduced and it becomes possible to improve a fuel consumption.
Further, under operating conditions of a high required load, by injecting the fuel in a intake stroke to lower the temperature of the intake gas in the combustion chamber, the charging efficiency can be improved. Further, the output can be improved by suppressing knocking.
Accordingly, in the art disclosed in for example Japanese Unexamined Patent Publication (Kokai) No. 61-250361 and Japanese Unexamined Patent Publication (Kokai) No. 61-250364, it is possible to simultaneously achieve both a low fuel consumption and a high output by switching the injection timing of the fuel between the compression stroke and the intake stroke in accordance with the required load conditions (degree of depression of accelerator pedal, engine speed, etc.)
When driving over a level road, the driver changes the degree of depression of the accelerator pedal (load) by only one driving pattern among three patterns, that is, to increase the speed of the vehicle from the current speed, to maintain the speed of the vehicle, and to lower the speed of the vehicle, therefore the accelerator pedal is not operated that frequently. Further, when the vehicle speed conspicuously changes, the combustion mode is simultaneously switched by the switching of the injection timing. Accordingly, the driver and passengers do not feel the slight switching shock such as the change of the vehicle speed occurring when the injection timing of the fuel and the combustion mode are switched as being uncomfortable.
However, when the inclination of the road changes during driving, that is, when the road goes up, levels off, or goes down, the driver has to operate the accelerator pedal frequently even when merely desiring to maintain the current vehicle speed. If the injection timing of the fuel and the combustion mode are switched at every operation of the accelerator pedal, many small changes in speed will occur even when driving at a constant speed. This shock is transferred to the human body each time resulting in an uncomfortable feeling in the driver and passengers.